1. Field of the Invention
The present invention relates generally to antennas, and in particular to a high-performance, multipath-rejecting antenna which forces correct polarization over a wide beamwidth including multiple Global Navigation Satellite System (GNSS) frequencies. A method of manufacturing such an antenna with a three-dimensional structure uses relatively inexpensive printed circuit board (PCB) production techniques.
2. Description of the Related Art
Various antenna designs and configurations have been produced for transmitting and receiving electromagnetic (wireless) signals. Antenna design criteria include performance considerations, such as the signal characteristics and the transmitters and receivers. Antenna manufacturing considerations include cost and compliance with manufacturing tolerances related to performance criteria. Antenna performance, cost and manufacturing considerations are important factors in connection with wireless devices in general, and particularly for GNSS receivers.
GNSSs include the Global Positioning System (GPS), which was established by the United States government and employs a constellation of 24 or more satellites in well-defined orbits at an altitude of approximately 26,500 km. These satellites continually transmit microwave L-band radio signals in three frequency bands, centered at 1575.42 MHz, 1227.60 MHz and 1176.45 MHz, denoted as L1, L2 and L5 respectively. All GNSS signals include timing patterns relative to the satellite's onboard precision clock (which is kept synchronized by a ground station) as well as a navigation message giving the precise orbital positions of the satellites. GPS receivers process the radio signals, computing ranges to the GPS satellites, and by triangulating these ranges, the GPS receiver determines its position and its internal clock error. Different levels of accuracy can be achieved depending on the techniques employed.
GNSS also includes Galileo (Europe), the GLObal NAvigation Satellite System (GLONASS, Russia), Compass (China, proposed), the Indian Regional Navigational Satellite System (IRNSS) and QZSS (Japan, proposed). Galileo will transmit signals centered at 1575.42 MHz, denoted L1 or E1, 1176.45 denoted E5a, 1207.14 MHz, denoted E5b, 1191.795 MHz, denoted E5 and 1278.75 MHz, denoted E6. GLONASS transmits groups of FDM signals centered approximately at 1602 MHz and 1246 MHz, denoted GL1 and GL2 respectively, and 1278 MHz. QZSS will transmit signals centered at L1, L2, L5 and E6. Groups of GNSS signals are herein grouped into “superbands.”
Multi-frequency capabilities provide several advantages. First, ionospheric errors can be corrected. Secondly, signals received on multiple frequencies can be averaged, thus reducing the effects of noise. Multipath errors from reflected signals also tend to be minimized with multi-frequency signal averaging techniques. Still further, an additional signal band(s) is available in case one frequency band is not available, e.g., from jamming.
Spiral-element and crossed-dipole antennas tend to provide relatively good performance for GNSS applications. They can be designed for multi-frequency operation in the current and projected GNSS signal bandwidths. Such antenna configurations can also be configured for good multipath signal rejection, which is an important factor in GNSS signal performance. An example of a crossed-dipole GNSS antenna is shown in Feller and Wen U.S. patent application Ser. No. 12/268,241, Publication No. US 2010/0117914 A1, entitled GNSS Antenna with Selectable Gain Pattern, Method of Receiving GNSS Signals and Antenna Manufacturing Method, which is incorporated herein by reference.
Multipath interference is caused by reflected signals that arrive at the antenna out of phase with the direct line-of-sight (LOS) signals. Multipath interference is most pronounced at low elevation angles, e.g., from about 10° to 20° above the horizon. They are typically reflected from the ground and ground-based objects. Antennas with strong gain patterns at or near the horizon are particularly susceptible to multipath signals, which can significantly interfere with receiver performance based on direct line-of-sight (LOS) reception of satellite ranging signals and differential correction signals (e.g., DGPS).
GNSS satellites transmit right hand circularly polarized (RHCP) signals. Reflected GNSS signals become left hand circularly polarized (LHCP) and are received from below the horizon as multipath interference, tending to cancel and otherwise interfere with the reception of line-of-sight (LOS) RHCP signals. Rejecting such multipath interference is important for optimizing GNSS receiver performance and accurately computing geo-referenced positions. Receiver system correlators can be designed to reject multipath signals. The antenna design of the present invention rejects LHCP signals, minimizes gain below the horizon and forces correct polarization (RHCP) over a relatively wide beamwidth for multiple frequencies of RHCP signals from above the horizon.
Previous GNSS antennas have addressed these design criteria. For example, prior art phasing networks were constructed with coaxial cables. However, precisely matching cable lengths tended to be difficult and expensive. Inductors and capacitors were also used in LC antenna circuits for delaying signals to achieve phase differencing. The tolerances of inductors and capacitors are difficult to maintain at these frequencies and are subject to stray capacitance and inductance due to the interconnections. A further prior art technique required two pairs of arms with resonances tuned off-center to create different phasing. However, the resulting bandwidths were relatively narrow and were susceptible to detuning by interference from the enclosure and other interference sources in the surrounding environment, such as the presence of ice and human contact.
Constructing precise phase-matching, multi-frequency, multipath-rejecting antenna systems with conventional prior art discrete components and manufacturing techniques tended to be relatively expensive, complicated and imprecise. Prior art antenna performance was compromised by imprecise phase-matching. Printed circuit board (PCB) materials and manufacturing techniques, on the other hand, are generally cost-effective and readily available. Moreover, PCBs can be etched to relatively tight tolerances. Maintaining such tolerances is important because the separate signal paths must be relatively precisely equal in length in order to avoid changing the phase differences or amplitudes of the signals before they reach the radiating elements, which are delayed 90° with respect to each other. Moreover, the signal paths need to be isolated from each other to avoid cross-path interaction and signal distortion.
The present invention addresses the aforementioned GNSS antenna design criteria by providing an antenna and manufacturing method using printed circuit board (PCB) materials and common manufacturing techniques.
Heretofore there has not been available an antenna and manufacturing method with the advantages and features of the present invention.